Thrust device, fuel injection valve having such a device, and method for manufacturing a thrust transfer element

ABSTRACT

A device for transferring a thrust, a fuel injection valve with such a device, and a method for manufacturing such a thrust transferring device. To transfer the thrust of an actuator, especially in an injection valve, mechanical tappets are used, which have substantially the shape of a cylinder whose defining surfaces are substantially triangular, with the corners rounded. By the great width of the tappet, broad contact areas are created so that friction during the transfer is reduced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority based on PCT ApplicationPCT/DE98/02887, filed Mar. 29, 1998, and German Application No.19742968.8, filed on Sep. 29, 1997, which are incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a device for transferring the thrust ofan actuator, an injection valve with such a device, and a method formanufacturing a tappet, such as may be used in such a device.

U.S. Pat. No. 4,101,076 to Bart discloses a conventional fuel injectionvalve which directly operates a fuel injector needle through amechanical tappet. The tappet has two lever arms of different length,which are arranged at right angles to one another and are joinedtogether at a bearing area. The shorter lever arm is associated with thepiezoelectric actuator and the longer lever arm with the injectorneedle. The tappet is fulcrumed in the bearing area on the housing ofthe fuel injection valve. The thrust of the piezoelectric actuator isconverted by the different lengths of the two lever arms to a greaterthrust of the injection needle.

These conventional valves suffer from a number of disadvantages,including the fact that their tappets are insufficiently stiff, whichleads to inaccuracy in the transfer of the thrust, especially in thecase of highly dynamic operations.

SUMMARY OF THE INVENTION

The present invention provides a device for transferring the thrust ofan actuator, which is simple in design and can be manufactured at lowcost. An important advantage of the present invention lies in the shapeof the tappet, which assures great rigidity, thereby ensuring thatthrust is directly transferred, even in highly dynamic processes.Another advantage of the present invention is that the tappets can bemanufactured with great accuracy and precision.

The present invention is achieved by providing a device for transferringthrust between an actuator and a plunger. The actuator moves a firstdisplacement relative to a body, and the plunger moves a seconddisplacement relative to the body. The device comprises a tappet havinga first end face, a second end face spaced along an axis from the firstend face, and a cylindrical surface surrounding the axis and extendingbetween the first and second end faces, the cylindrical surface having afirst contact portion adapted for contiguously engaging the body, asecond contact portion adapted for contiguously engaging the actuator,and a third contact portion adapted for contiguously engaging theplunger; and a guide having first and second surfaces, the first surfaceconfronting the first end face and the second surface confronting thesecond end face such that movement of the tappet along the axis issubstantially prevented.

The present invention is also achieved by providing an injector forsupplying fuel to a combustion chamber. The injector comprises a body;an actuator generating a thrust with respect to the body; a plungerdisplaceable with respect to the body by the thrust; a tappettransferring the thrust from the actuator to the plunger; and a guide.The tappet has a first end face, a second end face spaced along an axisfrom the first end face, and a cylindrical surface surrounding the axisand extending between the first and second end faces. The cylindricalsurface has a first contact portion contiguously engaging the body, asecond contact portion contiguously engaging the actuator, and a thirdcontact portion contiguously engaging the plunger. The guide has a firstsurface confronting the first end face and a second surface confrontingthe second end face, such that movement of the tappet along the axis issubstantially prevented.

The present invention is further achieved by providing a method ofproducing a tappet. The tappet has a first end face, a second end facespaced along an axis from the first end face, and a cylindrical surfacesurrounding the axis and extending between the first and second endfaces. The cylindrical surface has a first contact portion contiguouslyengaging a body, a second contact portion contiguously engaging anactuator, and a third contact portion contiguously engaging a plunger.The method comprises providing an elongated, right-circular cylindricalrod; machining the rod lengthwise such that a transverse cross-sectionalshape of the rod corresponds to a desired shape of the end faces; andcutting the rod transversely so as to provide a plurality of thetappets.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing, which is incorporated herein and constitutespart of this specification, illustrates presently preferred embodimentsof the invention, and, together with the general description given aboveand the detailed description given below, serves to explain features ofthe present invention.

FIG. 1 is a cross-section view of a fuel injection valve according tothe present invention having two tappets.

FIGS. 2A and 2B are schematic views illustrating the tappets and guideplate of the fuel injection valve shown in FIG. 1.

FIG. 3 is a cross-section view of a fuel injection valve according tothe present invention having a tappet.

FIG. 4 is a perspective view of a first tappet according to the presentinvention.

FIG. 4A illustrates certain features of the tappet shown in FIG. 4.

FIG. 5 is a perspective view of a second tappet according to the presentinvention.

FIG. 6 is a perspective view of a third tappet according to the presentinvention.

FIG. 7 is a perspective view of a fourth tappet according to the presentinvention.

FIG. 8 is a cross-section view of a fuel injection valve according tothe present invention having a plurality of tappets.

FIG. 9 is a schematic view of a fifth tappet according to the presentinvention.

FIG. 10 is a cross-section taken along the line X—X in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically a portion of a fuel injection valve with anactuator 1, which can be a piezoelectric actuator. The actuator 1 isplaced in a housing 2 and generates thrust against the bias of springs11, which are mounted in the housing 2. An upper end of the actuator 1is in contact with a cover plate of the housing 2. And the second end ofthe actuator 1 lies against an actuator piston 10 which is ofcylindrical shape and is guided in a cylindrical recess in the housing2.

The housing 2 is inserted into an opening 4 of a valve body 3 andtightly fastened by screws to the valve body 3. In the opening 4 thereis a spacer ring 9 on which the housing 2 is supported. The opening 4has a central, circular recess 5 is connected by a bore 20 to a valveseat 19 and a control chamber 18. Within the bore 20, a plunger 6, inthe form of a piston, is guided with a tip extending through the valveseat 19. A valve body 7 at the end of the tip is in contact with thevalve seat 19 and seals the control chamber 18 against the bore 20.

Between the actuator piston 10 and the plunger 6 are two mechanicaltappets 14 and 22. The two tappets 14 and 22 are arranged symmetricallyon opposite sides of the central axis 21. The tappets 14,22 are ofidentical shape and placed in the recess 5. The recess 5 has an annularbearing surface 8 that is formed by the valve housing 3, and is disposedsubstantially perpendicular to the direction of movement of the plunger6 and parallel to the thrusting face of the actuator piston 10. Theactuator piston 10 and the plunger 6 are symmetrical with one anotherand centered on the central axis 21. Of course, one or more than twotappets may be symmetrically positioned around the central axis 21.

Each of the tappets 14,22 has a first contact area 15 lying on thefulcrum surface 8. Also, each tappet 14,22 has a third contact area 17that lies on the top of the plunger 6. Furthermore, a second contactarea 16 of each tappet 14 is in contact with the actuator piston 10. Thesecond contact area 16 lies between the first and third contact areas15,17, while the first and third contact areas 15 and 17 are disposed onthe bottom of the tapped, and the second contact area 16 is disposed onthe top of the tappet 14.

The tappets 14,22 are fixed in their position perpendicular to themovement direction of the actuator 10 and of the plunger 6 by the recess5 and by the spacer ring 9. The spacer ring 9 has a through-opening inwhich the tappets 14,22 are placed such that the first and the secondtappets 14,22 are arranged in symmetry with one another, with thecentral axis 21, and with the actuator piston 10 and the plunger 6.

FIG. 2A shows a schematic top view of the arrangement of the actuatorpiston 10 with respect to the first and the second tappets 14,22 and theplunger 6. The actuator piston 10 has a larger diameter than the plunger6. Also shown is the circular opening 4 and the recess 5 symmetricalwith the opening 4. The recess 5 can have the same shape as the opening4, e.g., a circular shape. In the recess 5, the first and the secondtappets 14,22 are positioned symmetrically with an axis of symmetry Mthat passes through the central axis 21.

In FIG. 2A, the spacer ring 9, which is shown hatched, is of circularshape and its outside circumference corresponds substantially to theopening 4.

The spacer ring 9 is shown again more clearly in FIG. 2B. The spacerring 9, which also functions as a guide disk, has an adjustment recess24 that has a substantially rectangular shape with a centrally locatedcircle superimposed thereon. The rectangular shape guides and maintainsthe first and second tappets 14,22 in symmetry with one another on oneaxis. The central circular recess enables the round actuator piston 10to move unhampered toward the first and second tappets 14,22. Therefore,the circular portion of the adjusting recess of the spacer ring 9 is atleast as large as the cross-section of the actuator piston 10.

The operation of the system represented in FIG. 1 will now be described.When the piezoelectric actuator 1 is operated, the actuator 1 elongatestoward the plunger 6 against the force of the springs 11. The actuatorpiston 10 thus presses against the second contact surface 16 of thefirst and second tappet 14,22. The first and second tappets 14,22 areeach in contact at a first contact area 15 with the fulcrum surface 8 ofthe valve housing 3. Also, the first and second tappets 14,22 have athird contact surface 17 in contact with the plunger 6. The plunger 6 ismovable in the bore 20. The pressure of the actuator piston 10 causesthe first and second tappets 14,22 to thrust with the first contact area15 against the valve body 3 and move into a working position in whichthe plunger 6 is driven by the third contact area 17 toward the valve 7.

In this manner the valve 7 is moved apart from the associated valve seat19 and the control chamber 18 is opened, so that fuel, for example,which is present under high pressure in the control chamber 18, can flowthrough the bore 20 to an outlet. Simultaneously fuel flows through afirst throttle and a feed passage 12 at high pressure into the controlchamber 18. With the valve seat 19 open, however, more fuel flows downthan flows through the throttle. Thus the pressure in the controlchamber drops. In the control chamber 18, an adjusting piston 23 isprovided which transfers the pressure in the control chamber 18 to avalve needle. The valve needle is associated with a fuel injector and,depending on the pressure in the control chamber, opens or closes thefuel injector for injecting fuel into an internal combustion engine. Theinjector is also connected to a passage 12 which carries fuel under highpressure.

When power to the actuator 1 is shut off, the actuator 1 shortens and isurged upward by the springs 11 and by the first and second tappets 14,22toward the top cover of the housing 2, i.e., toward its initialposition. The pressure in the control chamber 18 forces the valve 7,which can be spherical in shape, against the valve seat 19 andsimultaneously forces the plunger 6 toward the first and the secondtappets 14,22. Thus, the first and the second tappets 14,22 are returnedto the starting position.

The first and second tappets 14,22 pivot about centers which are locatedover the first contact area 15 of the first and second tappets 14,22.Since the second contact area 16 at which the actuator piston 10 lies onthe first and second tappets 14,22 is at a shorter distance from thosecenters than from the third contact area 17, the lengthening of theactuator is converted to a greater motion of the plunger 6.

An especially precise and easy operation of the plunger 6 is achieved ifthe first and the second tappets 14,22 are adjusted precisely to oneanother, to the actuator piston and to the plunger 6 by the spacer ring9. Preferably, by an appropriate selection of the thickness of thespacer ring 9, an adjustment of the space between the actuator piston 10and the first and second tappets 14,22 is achieved, and thus also anadjustment of the space between the actuator piston 10 and the plunger6. The precise adjustment of the first and of the second tappets 14,22,which also does not interfere with the actuator piston 10, is achievedessentially by the adjusting recess 24 which is located in the spacerring 9. What is important is the special shape of the adjusting recess24, which can be formed as a rectangle with a centrally superimposedcircle. In a simple embodiment of the spacer ring 9, the adjustingrecess is configured only as a quadrangular recess.

FIG. 3 shows an additional embodiment of a fuel injection valve in whichonly a single tappet 14 is provided in order to transfer the thrust ofthe actuator 1. The tappet 14 lies with its first contact area 15 on thefulcrum surface 8 of the valve body 3 and is carried by the guiding ring9. The second contact area 16 is in contact with the actuator piston 10.The third contact area 17 of the tappet 14 lies on the center of theplunger 6. The arrangement in FIG. 3 operates the same as thearrangement in FIG. 1, except that the thrust of the actuator 1 istransferred to the plunger 6 through a single tappet 14.

In FIG. 3, “D” indicates the center of rotation of the tappet 14. Thecenter D is situated vertically above the first contact area 15. Thesecond contact area 16 has a first effective distance a from center D.The expression “effective distance,” as it is used in connection withthe present invention, means the distance that is important to a leveraction. The third contact area 17 is at a second effective distance bfrom the center D. The leverage ratio Ü is defined by Ü=a/b. The firstand the second effective distances a and b are established by the shapeof the tappet, and thus the leverage ratio as well.

Instead of the arrangement with a single tappet according to FIG. 3, orthe arrangement with two tappets according to FIG. 1, a greater numberof tappets can also be used. However, for a low-friction operation ofthe plunger 6, it is necessary that the thrust of the actuator piston 10be applied centrally or with a uniform distribution to the plunger 6, soas to prevent any skewing of the plunger 6. For that purpose it isnecessary to align the tappets 14 and 22 symmetrically with the centralaxis 21 passing centrally through the plunger 6.

FIG. 4 is a perspective view of a tappet 14. The tappet 14 hasessentially the shape of a cylinder whose circumference FM isperpendicular to the end faces FG. The end faces FG are identical inshape. The end face FG is advantageously of a triangular shape whereinthe corners are rounded.

In shaping the tappet 14, it is essential that the width B be exactly asgreat as the length L of the tappet. Due to the relatively great widthB, elongated contact surfaces are achieved in the first, second andthird contact areas 15,16,17, which are narrow but relatively long.Relatively large contact surfaces are thus achieved in the first, secondand third contact areas 15,16,17. Thus, single-point application ofpressure is avoided and a broad application of pressure is achieved atthe actuator piston 10, the contact surface 8, the plunger 6 and thetappets 14 and 22.

The width of the contact surfaces 15,16,17 is established essentially bythe curvature of the tappets 14 and 22 in their corresponding areas. Theshape of the contact areas 15,16,17 is chosen such that, on the onehand, the leverage ratio is achieved, and on the other hand, wear byfriction or seizure due to excessively great surface pressure will beminimized. The curvature in the area of the contact surfaces 15,16,17 isto be as small as possible. Also, the tappet 14 should be as high aspossible so as to achieve as much rigidity as possible. The relativeratios of the height H, length L and width B of the tappet 14 areimportant, and is to be chosen such that sufficient rigidity is achievedfor the given leverage.

The tappet 14 is slightly recessed on the bottom between the first andthe third contact areas 15,17, so that the tappet 14 will definitelythrust against the fulcrum surface 8 and the actuator piston 6 only atthe first and third contact areas 15,17, and also permit the tappet 14to turn about the pivot center D without striking the valve body 3 in adifferent area.

Referring again to FIG. 4, the first, second and third contact areas15,16,17 are represented with a broken line. The contact areas 15,16,17are substantially in the form of a linear contact at which the first,the second and the third contact areas engage the actuator piston 10,the plunger 6 and the fulcrum surface 8, respectively. The distancebetween the first and third contact areas 15,17 is referred to as thelever width A. If the first and the third contact areas 15,17 lie on aflat plate, the lever height H is defined by the distance between thesecond contact area 16 and the flat plate. Tests have shown thatsufficient rigidity is achieved in the tappet 14, assuming that there islittle frictional movement of the lever at the contact areas, if theratio of the lever height H to the lever width A (H:A) is less than orequal to 1. A preferred ratio of the lever height to the lever width H:Aranges between 1:4 and 3:4.5. With these dimensions, sufficient rigidityis achieved and, at the same time, the relative movement of the tappet14 when the piezoelectric actuator stretches is reduced to a necessaryminimum. In addition, tests have shown that the tappet 14 can have awidth B of preferably 2 to 13 mm.

The stress on the tappet 14 is further reduced if the first, second andthird contact areas 15,16,17, which are essentially contact lines, areas parallel as possible to one another. A preferred maximum departure inthe parallelism between any two f the first, second and third contactlines is in the range of 50 μm, with a preferred lever width of up to 6mm. The wear on the tappet 14 is further reduced if the first, secondand third contact areas 15,16,17, which are configured as contact lines,have a parallelism deviation W between two contact lines which is lessthan 10 μm for a lever width of 6 mm. This is represented schematicallyin FIG. 4A.

Advantageously, the friction that occurs on the tappet 14 can be reducedif the surfaces of the tappet 14, especially in the area of the first,second and third contact areas 15,16,17 have a high surface qualityvalue R_(z) that is preferably less than 1 μm. R_(z) is the rough depthaccording to DIN Standard 4768. Also, with a hardness of more than 60HRC (Rockwell hardness), a tappet can be made that will withstand greatstresses.

The friction forces which attack the tappet 14 are also reduced bylubrication such as, for example, having the tappet run in the fuelleakage of the injector.

An additional advantageous embodiment of the tappet 14 consists inproviding at least the first, second and third contact areas 15,16,17with a surface coating that reduces friction in the movement of thetappet 14 and also satisfies the stringent requirements of surfacequality and hardness. Preferably a titanium nitride layer serves as thatcoating. The tappet 14 can be made of a very stiff material with amodulus of elasticity that is greater than that of steel, for example,greater than 500,000 N/mm².

Furthermore, tests have shown that the rounding radii of the first,second and third contact areas 15,16,17 can range from 5 to 30 mm. Ithas been found that it is advantageous if the ratio between the roundingradius R1,R2 and the force with which the first, second and thirdcontact areas are applied to a contact point is as follows:radius/contact force≦7.5 mm/1000 Newtons. This ratio advantageouslyreduces wear on the tappet 14.

The above-given dimensions of the tappet 14 are not limited solely tothe embodiment represented in FIG. 4, but are also advantageous in otherforms, as for example those represented in FIGS. 5 and 6.

An advantage of using only one tappet 14, as shown in FIG. 3, is thatthe manufacturing costs are lowered and the amount of space required isreduced.

FIG. 5 shows an advantageous form of the tappet 14. The essential shapeof the tappet 14 is that of a cylinder whose defining surfacescorrespond largely to a triangle with rounded corners. Also, the tappet14 is made as wide as possible perpendicular to the direction ofrotation, i.e., the cylinder is relatively long for the size of thedefining surfaces. The rigidity of the tappet is thereby increased. Thefirst, second and third contact areas are each provided with a curvatureof a given radius. These areas substantially constitute the shape of acylinder. The radius of curvature is preferably identical in the first,second and third contact areas 15,16,17.

The tappet of FIG. 5 has an upper, preferably planar surface 40extending from a first rounded area 41 to a second rounded area 42. Thefirst and the second rounded areas 41 and 42 are associated lengthwisewith the cylindrical shape of the tappet. The second rounded area 42 isdisposed above the third contact surface 17. The first rounded area 41is between the first and the second contact surface 17. The tappetsurface 40 is inclined upwardly toward the second rounded area 42, i.e.,slopes upwardly. The tappet surface 40 merges at the two rounded areas40 and 41 with a curvature with the radius R1. The function of theinclined surface 40 is based on the fact that the actuator piston in therest position lies on the second rounded area 42, which is higher thanthe first rounded area 41. Thus the second contact area 16 is disposedon the second rounded area 42. The third contact area 17 is underneaththe second contact area 16, and the third and second contact area arethe same lateral distance from the first contact area 15. When theactuator piston 10 is driven, the motion is transferred directly, i.e.,in direct ratio, through the third contact area 17 to the associatedplunger 6.

When the actuator piston 10 is driven further, the second contact area16 shifts on the tappet surface 40 toward the first rounded area 41.Thus, the lateral distances at which the second and the third contactarea 16 and 17 are situated from the first contact area 15 aredifferent, i.e., the distance of the second contact area 16 decreasesand the distance of the third contact area 17 remains the same. Thus,the motion of the actuator piston 10 is converted to a greater motion ofthe plunger 6. The leverage ratio between the motion of the actuatorpiston 10 and the motion of the plunger 6 increases as the motion of theactuator piston 10 increases. It is especially advantageous if, at thestart of the motion of the actuator piston 10, the leverage ratio is oneand then the leverage ratio increases.

The ratio between the movement of the actuator 1 and the movement of theplunger 6 is established by the upward slope of the tappet surface 40.

Thus, at first the valve 7 is driven away from the valve seat 19 withgreat force and a low ratio. For this action an application of greatforce is necessary. This is achieved by the direct ratio. If the valve 7is lifted from the valve seat the pressure in the control chamber 18decreases. At low pressure the force needed for the further opening ofthe drain is less, and a rapid opening of the drain increases thedynamic of the servo valve, so that a stepping up of the motion of theactuator piston advantageously occurs. On account of the variable ratioof the motion, the actuator 1 can be optimally adapted to its power. Inaddition, the dynamic is increased.

FIG. 6 shows an additional advantageous embodiment of the tappet, inwhich the first and second rounded areas 41,42 of the tappet surface 40are closer together, and the pitch with which the tappet surface 40rises from the first to the second rounded area is steeper. Thus thestep-up ratio increases more rapidly with the motion of the actuatorpiston than in the embodiment in FIG. 5. Also, the maximum step-up ratiois lower than in FIG. 5, since the distance of the first rounded portion41 from the second rounded portion 42 is less than in FIG. 5.Preferably, the pitch of the tappet surface 40 can also be equal to thepitch of the tappet surface 40 in the embodiment in FIG. 5.

FIG. 7 shows an additional advantageous embodiment of a tappet 14 whichis machined from a cylindrical rod. First, the shape of the bottom side,which is between the first and third contact areas 15,17, is machinedinto the rod 25. Then the shaped rod is sliced into individual pieces.Thus, the pieces of the rod, which constitute the tappets 14, are madeidentical in shape and especially identical in the first, the second andthe third contact areas 15,16,17. A slight variation in the thickness ofthe tappets is less important to an accurate and precise operation ofthe plunger 6.

Preferably, the first contact area 15, the third contact area 17, andthe recessed area 26 that is created in the tappet 14 on the undersideof the tappet 14, have the same radius R2. In a further embodiment, thefirst and the third contact areas have a first radius and the recessedarea 26 has a second, larger radius.

The shape is advantageously made in the rod 25 by grinding. The severingof the pieces is performed, for example, by wire erosion, laser cuttingor electron-beam cutting. In a further embodiment, before the pieces aresevered, the shape, i.e., the curvature of the second contact area 16,is made in the rod 25 if it is to be different from the curvature of therod 25.

FIG. 8 shows another injection valve in which third tappets 30 areprovided between the actuator piston 10 and the plunger 6. The plunger 6is biased by leaf springs 31 toward the third tappet 30 and the actuatorpiston 10.

FIG. 9 shows a top view of the third tappets 30 with an adjusting device32. The adjusting device 32 is configured as a surrounding ring out ofwhich guiding projections 33 point toward the center of the circle.Between each pair of guiding projections 33 a third tappet 30 isinserted.

The third tappet 30 is made essentially in the form of a flat platewhich is slightly curved in the areas in which the actuator piston 10,the plunger 6 or the fulcrum surface 8 are engaged, so that the frictionis reduced.

FIG. 10 shows the rounded first, second and third contact areas 15,16,17of the tappet 30 in cross-section.

While the invention has been disclosed with reference to certainpreferred embodiments, numerous modifications, alterations, and changesto the described embodiments are possible without departing from thesphere and scope of the invention, as defined in the appended claims andtheir equivalents thereof. Accordingly, it is intended that theinvention not be limited to the described embodiments, but that it havethe full scope defined by the language of the following claims.

What is claimed is:
 1. A device for transferring thrust between an actuator and a plunger, the actuator moving a first displacement relative to a body and the plunger moving a second displacement relative to the body, the device comprising: a tappet having a first end face, a second end face spaced along an axis from the first end face, and a cylindrical surface surrounding the axis and extending between the first and second end faces, the cylindrical surface having a first contact portion adapted for contiguously engaging the body, a second contact portion adapted for contiguously engaging the actuator, and a third contact portion adapted for contiguously engaging the plunger, the first contact portion including a circumferential segment of a first right circular cylinder having a first radius, the second contact portion including a circumferential segment of a second right circular cylinder having a second radius, and the third contact portion including a circumferential segment of a third right circular cylinder having a third radius, the first, second, and third radiuses being equal; and a guide having first and second surfaces, the first surface confronting the first end face and the second surface confronting the second end face such that movement of the tappet along the axis is substantially prevented.
 2. The device according to claim 1, wherein the tappet pivots parallel to the axis.
 3. The device according to claim 1, wherein the first and second end faces are substantially parallel, and the first and second surfaces are substantially parallel.
 4. The device according to claim 1, wherein a ratio of a first effective distance between centers of rotation of the first contact portion and the second contact portion to a second effective distance between centers of rotation of the second contact portion and the third contact portion substantially equals a ratio of the first displacement to the second displacement.
 5. The device according to claim 1, wherein at least one of the first, second, and third contact portions extends between the first and second end faces.
 6. The device according to claim 5, wherein each of the first, second, and third contact portions extend between the first and second end faces.
 7. The device according to claim 1, wherein a plurality of tappets are arranged symmetrically about a line of action coincident with the first and second displacements, and each of a corresponding plurality of guides prevent axial movement of a respective one of the plurality of tappets.
 8. The device according to claim 7, wherein a pair of tappets are arranged diametrically opposite the line of action, and a disk surrounding the line of action comprises a pair of guides, each of the pair of guides receives a respective one of the pair of tappets.
 9. The device according to claim 7, wherein a ring circumscribes the plurality of tappets and connects each of the plurality of guides.
 10. The device according to claim 9, wherein the ring includes a corresponding plurality of wedge-shaped elements having respective apexes pointing toward the line of action, each of the plurality of wedge-shaped elements providing the first surface confronting the first end face of a first tappet and the second surface confronting the second end face of a second tappet, the first and second tappets being circumferentially adjacent to one another, and the first and second surfaces of each of the plurality of wedge-shaped elements intersecting one another at the respective apex.
 11. A device for transferring thrust between an actuator and a plunger, the actuator moving a first displacement relative to a body and the plunger moving a second displacement relative to the body, the device comprising: a tappet having a first end face, a second end face spaced along an axis from the first end face, and a cylindrical surface surrounding the axis and extending between the first and second end faces, the cylindrical surface having a first contact portion adapted for contiguously engaging the body, a second contact portion adapted for contiguously engaging the actuator, and a third contact portion adapted for contiguously engaging the plunger, at least one of the first, second, and third contact portions being a circumferential segment of a right circular cylinder, the second contact portion including a first circumferential segment of a first right circular cylinder having a first radius, a second circumferential segment of a second right circular cylinder having a second radius, and a connecting portion extending between the first and second circumferential segments, the first and second radiuses being equal; and a guide having first and second surfaces, the first surface confronting the first end face and the second surface confronting the second end face such that movement of the tappet along the axis is substantially prevented.
 12. The device according to claim 11, wherein the tappet pivots parallel to the axis.
 13. The device according to claim 11, wherein the first and second end faces are substantially parallel, and the first and second surfaces are substantially parallel.
 14. The device according to claim 11, wherein a ratio of a first effective distance between centers of rotation of the first contact portion and the second contact portion to a second effective distance between centers of rotation of the second contact portion and the third contact portion substantially equals a ratio of the first displacement to the second displacement.
 15. The device according to claim 11, wherein at least one of the first, second, and third contact portions extends between the first and second end faces.
 16. The device according to claim 15, wherein each of the first, second, and third contact portions extend between the first and second end faces.
 17. The device according to claim 11, wherein a plurality of tappets are arranged symmetrically about a line of action coincident with the first and second displacements, and each of a corresponding plurality of guides prevent axial movement of a respective one of the plurality of tappets.
 18. The device according to claim 17, wherein a pair of tappets are arranged diametrically opposite the line of action, and a disk surrounding the line of action comprises a pair of guides, each of the pair of guides receives a respective one of the pair of tappets.
 19. The device according to claim 17, wherein a ring circumscribes the plurality of tappets and connects each of the plurality of guides.
 20. The device according to claim 19, wherein the ring includes a corresponding plurality of wedge-shaped elements having respective apexes pointing toward the line of action, each of the plurality of wedge-shaped elements providing the first surface confronting the first end face of a first tappet and the second surface confronting the second end face of a second tappet, the first and second tappets being circumferentially adjacent to one another, and the first and second surfaces of each of the plurality of wedge-shaped elements intersecting one another at the respective apex.
 21. An injector for supplying fuel to a combustion chamber, the injector comprising: a body; an actuator generating a thrust with respect to the body; a plunger displaceable with respect to the body by the thrust; a tappet transferring the thrust from the actuator to the plunger, the tappet having a first end face, a second end face spaced along an axis from the first end face, and a cylindrical surface surrounding the axis and extending between the first and second end faces, the cylindrical surface having a first contact portion contiguously engaging the body, a second contact portion contiguously engaging the actuator, and a third contact portion contiguously engaging the plunger, the first contact portion including a circumferential segment of a first right circular cylinder having a first radius, the second contact portion including a circumferential segment of a second right circular cylinder having a second radius, and the third contact portion including a circumferential segment of a third right circular cylinder having a third radius, the first, second, and third radiuses being equal; and a guide having first and second surfaces, the first surface confronting the first end face and the second surface confronting the second end face such that movement of the tappet along the axis is substantially prevented.
 22. The injector according to claim 21, wherein a plurality of tappets are arranged symmetrically about a line of action coincident with displacement of the plunger, and each of a corresponding plurality of guides prevent an axial movement of a respective one of the plurality of tappets.
 23. An injector for supplying fuel to a combustion chamber, the injector comprising: a body; an actuator generating a thrust with respect to the body; a plunger displaceable with respect to the body by the thrust; a tappet transferring the thrust from the actuator to the plunger, the tappet having a first end face, a second end face spaced along an axis from the first end face, and a cylindrical surface surrounding the axis and extending between the first and second end faces, the cylindrical surface having a first contact portion contiguously engaging the body, a second contact portion contiguously engaging the actuator, and a third contact portion contiguously engaging the plunger, the second contact portion including a first circumferential segment of a first right circular cylinder having a first radius, a second circumferential segment of a second right circular cylinder having a second radius, and a connecting portion extending between the first and second circumferential segments, the first and second radiuses being equal; and a guide having first and second surfaces, the first surface confronting the first end face and the second surface confronting the second end face such that movement of the tappet along the axis is substantially prevented.
 24. The injector according to claim 23, wherein a plurality of tappets are arranged symmetrically about a line of action coincident with displacement of the plunger, and each of a corresponding plurality of guides prevent an axial movement of a respective one of the plurality of tappets. 